1. Field of the Invention
The present invention relates to an apparatus for protecting impact to a rack bar of a power steering gearbox, and more particularly an apparatus for protecting impact to a rack bar of a power steering gear box, in which impact and noise are not generated by releasing a part of oil pressure immediately before a piston of the rack bar moving by the power of oil pressure reaches a rack bush.
2. Description of the Related Art
In general, an automobile is provided with a power steering device for light and rapid steering operation, which comprises an oil pump to generate oil pressure, a power source, by driving an engine of the automobile, a control valve to control oil supplied from the oil pump by opening and closing a oil pathway according to operation of a steering wheel, and a gearbox to generate a steering power by the oil pressure transmitted through the control valve.
FIG. 1 is a front view of a gearbox and a control valve of a conventional power steering device. This prior art will be explained with reference to Korean Laid-Open Utility Model Publication No. 2000-0016760 and Korean Laid-Open Patent Publication No. 2001-0027557.
As shown in FIG. 1, a gearbox 20 elongated from side to side to be connected with a wheel is provided at the center with a control valve 10 which is installed to cross the axis of the gearbox 20. Also, the control valve 10 is connected with the gearbox 20 by oil pressure tubes 40.
The control valve has a valve housing 11 to define the outward appearance. The valve housing is provided with oil ports 12, through which oil flows, on the outside thereof and an input axis 13 at the center thereof.
Inside the input axis 13 installed within the valve housing 11, a rotary valve, though it is not shown, is disposed to supply oil pressure to the inside of the gearbox 20 by controlling the oil pathway. By this rotary valve, the gearbox 20 generates assistant servo by oil pressure to achieve power steering.
At the lower part of the control valve 10, the gearbox is disposed to generate a steering power by moving a tie rod 25 extending from side to side by oil pressure supplied to the control valve 10. The gearbox has a shape of an elongated cylinder 26 with both sides opened. The cylinder 26 is provided at the inside of one end with a nut part 29 having screw threads so that a rack bush 50 can be installed. Also, the cylinder 26 is provided with a rod-shaped rack bar 21, which extends while passing through both ends of the cylinder 26 and can slide toward both ends of the cylinder 26. The rack bar 21 is joined to the tie road which is connected at its both sides extending out of the cylinder 26 with the wheel side and thus, it is possible to directly manipulate wheels. On the outer circumference of the cylinder, a left port 23 and right port 24 are provided to be connected with the inside of the cylinder so that the oil through the control valve 10 can be supplied to the inside of the cylinder 26.
On the outer surface of the rack bar 21 which is located within the cylinder 26, a disc-shaped piston 27 is provided, which separates the inner space of the cylinder 26 and the rack bar 21 can operate by the oil pressure transmitted through the piston.
The cylinder 26 is provided with oil seals 30 inside the open ends to prevent oil leakage. The outer circumference of the oil seals 30 are fixed at the inner surface of the cylinder 26 and the inner circumference contacts with the rack bar 21 in a way that the rack bar 21 can slide. Also, the rack bar 21 is provided on the outer surface of its one side with rack gears (not shown) extending from side to side and the input axis 13 is provided with pinion gears (not shown) which engage with the rack gears to directly transmit rotation power to the rack gears, whereby the rack gears can move from right to left.
Meanwhile, the cylinder 26 is provided at its one end with a rack bush 50 which guides the movement from side to side of rack bar 21 and prevents noise generated inside the cylinder 26 and oil leakage. The rack bush 50 is a cylindrical hollow bar having a hollow part 54, through which the rack bar 21 can slide.
Outside the hollow part 54, there are an insert part 52 which is inserted in the cylinder 26, a flange part 51 having a diameter greater than that of the input part which is disposed toward the open end of the cylinder 26 to close the end of the cylinder, and a bolt part 53 having screw threads which is disposed on the outer circumference of the insert part next to the flange part 51. The bolt part 53 is fastened to the nut part 29 formed at one end of the cylinder 26 to join the rack bush 50 with the cylinder 26.
Thus, in case of the conventional power steering apparatus constructed as described above, oil supplied from the oil pump to the cylinder circulates while a driver does not manipulate the steering wheel. When the driver manipulates the steering wheel the pinion gears rotate and thereby rack gears engaged with the pinion gears move in a predetermined direction. At the same time, the rotary valve disposed in the input axis 13 controls an oil pathway, whereby oil is supplied to the cylinder 26 so that the rack bar 21 can move to the direction along which the steering wheel rotates. In this way, the piston 27 slides and the driver can readily turn the steering wheel. At this time, the rack bar 21 slides from the left side to the right side within the cylinder 26 and rack bush 30 is disposed at one side inside the cylinder 26 to guide such movement of the rack bar 21.
However, such conventional apparatus has problem in that when the piston 27 moves to either right or left end by the action of oil pressure and touches the rack bush 50 located at the end, impact and thereby noise can be generated, since the oil pressure is too great, causing damage to stability.